All-or-none Response: Phenomenon in which a muscle fiber contracts completely when exposed to a threshold stimulus, or not at all. When a skeletal muscle is stimulated to contract by a motor neuron a minimum amount of stimulus is needed to start the process of muscle contraction.
This minimum amount of stimulus is called the threshold stimulus and results in an action potential. Once this threshold signal is released, a muscle cell contracts completely. A muscle cell never contracts partially. Once the stimulus reaches the depolarization threshold, the muscle membrane depolarizes and the cell (fiber) contracts.
The all or none response is the same for nerves cells, where if a threshold stimulus for depolarization is reached, the membrane depolarizes. Any increase in the intensity of the stimulus does not result in a greater response. Either a muscle or nerve cell depolarizes completely or not at all..so "all" or "none at all". In this way, a muscle fiber always contracts maximally, even if the contraction is initiated by a large electrical shock rather than a motor neuron signal..the contraction is the same intensity, either maximal or nonexistent.
A motor neuron and the muscle fibers it controls is called a "motor unit". Even though a motor unit, when a threshold is reached, contracts maximally this does not keep the body from controlling the strength of its various movements. Instead of the intensity of the contraction of individual motor units changing in response to different needs, it is the number of motor units firing at any one time that controls the strength of the response. One motor unit may be stimulated when one is picking up a pencil or pen, and hundreds when one is picking up a heavy dumbbell. Thus, muscle effort is regulated by this graded response or orderly muscle recruitment.
This page created 13 Oct 2011 17:07
Last updated 27 Feb 2016 23:02